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Sharing is thought to be a unique human trait developed from social interactions and our ability to use language, but a new study published today, Jan. 2, by the journal PLOS ONE finds that bonobos share with strangers to increase their social circle.

"Our results show that generosity toward strangers is not unique to humans. Like chimpanzees, our species would kill strangers; like bonobos, we could also be very nice to strangers," says lead author Jingzhi Tan of Duke University in a press release. "Our results highlight the importance of studying bonobos to fully understand the origins of such human behaviors," he added.

The researchers placed bonobos living in the Lola ya Bonobo sanctuary in Kinshasa in the Democratic Republic of Congo in certain situations in which they had an opportunity to share food with each other. Bonobos helped one another acquire food that was out of reach, but, interestingly, they only shared their own food with strangers — unrelated bonobos from different social groups.

"It seems kind of crazy to us, but bonobos prefer to share with strangers," said co-author Brian Hare, a professor of evolutionary anthropology at Duke University in a press release. "They're trying to extend their social network." And they apparently value that more than maintaining the friendships they already have.

Without seeing a social benefit, the bonobos refused to share food. This means that sharing between humans may have developed to expand our social networks and not because of language, as originally thought.

"They care about others," Hare said, but only in a sort of selfish way. "They'll share when it's a low-cost/low-benefit kind of situation. But when it's a no-benefit situation, they won't share. That's different from a human playing the dictator game. You really have to care about others to give anonymously."